1. Field of the Invention
The present invention relates in general to a blood pressure monitor device, and more particularly to such a device which iteratively measures a blood pressure of a subject during a pressing force decreasing operation.
2. Related Art Statement
There is known a blood pressure monitor system including a pressing device such as an inflatable cuff for pressing a body portion of a subject; and a measuring device which increases the pressing force of the pressing device, subsequently decreases the pressing force, and measures a blood pressure of the subject based on Korotkoff sounds or pulse wave obtained during decreasing of the pressing force, the measuring device repeating the blood pressure measurement at predetermined regular intervals of time. In the conventional monitor system, the measuring device increases the pressing force to a target pressure which is a predetermined high level (e.g., 180 mmHg), and subsequently reduces the pressing force at a low rate (e.g., 2 to 3 mmHg/sec). However, the measuring device is incapable of measuring a systolic blood pressure of the subject which is higher than the target pressure. In this case, the measuring device is required to increase the pressing force to a level higher by a suitable amount than the systolic blood pressure. To prevent this problem, another known monitor system is adapted to increase the pressing force to a target pressure which is higher by a predetermined amount than a systolic blood pressure obtained in a preceding blood pressure measuring cycle.
However, in the above described second monitor system, the target pressure is constantly set at the level higher by a predetermined pressure than the previously obtained systolic blood pressure, without considering variety of subjects. Therefore, for some subjects, the target pressure may be unnecessarily high. Consequently, those subjects may feel uneasiness or pain, and the time necessary for the pressing force increasing operation may be unnecessarily long, which leads to increasing the time necessary for the blood pressure measurement. More specifically, in the blood pressure monitoring system of the Korotkoff-sound type, a Korotkoff sound is detected when the pressing force which has been increased to the target pressure is reduced to the level of systolic blood pressure. However, the detected sound may not be the first Korotkoff sound, but a noise signal. For judging whether or not the detected sound is the true, first Korotkoff sound, the system is required to continue reading in the signal for a predetermined length of time corresponding to, for example, at least three sounds. On the other hand, the monitor system of the oscillometric type determines, as a systolic pressure, a pressing force (e.g., cuff pressure) at the time of the first inflection point of the amplitudes of pulses detected during the pressure reducing operation. However, for identifying the first inflection point, the system is required to continue reading in the signal for a predetermined length of time corresponding to, for example, at least five pulses which include the inflection point as a middle data point or pulse. Therefore, the reading time necessary for reading in the signal or data for detecting the first Korotkoff sound or the first inflection point varies depending upon pulse rates of subjects. Namely, the reading time for a subject whose pulse rate is fast is short, while the time for a subject whose pulse rate is slow is long. For this reason, in the conventional monitor systems, the predetermined amount to be added to the prior systolic blood pressure is required to be a sufficiently large value (e.g. 50 mmHg) so as to provide a reading time long enough for the subject whose pulse rate is slow. On the other hand, for the patient whose pulse rate is fast, the target pressure thus determined becomes unnecessarily high, whereby the patient may feel uneasiness or pain and the time necessary for the blood pressure measurement is unnecessarily increased.